1. Field of the Invention
The present invention relates generally to an exit window for an X-ray lithography beamline, and more particularly, to a thin, cylindrically shaped beryllium exit window that is attenuating and can resist a vacuum pressure differential of at least 14.7 psi. The hoop stress effect makes it possible for the thin foil to be wide enough to accommodate the full scan of the lithography beam as a stationary window.
2. Description of the Prior Art
Since the first planar transistor was fabricated in the late 1950's the number of transistors on integrated circuits have doubled every year. The driving force behind this growth has been the ever increasing demand for higher speed communication in ever increasing chip sizes. The continuously increasing production and resolution requirements for manufacturing semiconductor devices will lead to performing microlithographic operations on 8-10 inch diameter wafers producing lithographed lines with 0.25 micron resolution in the mid 1990s. To meet the demands, these operations should be performed at a high production rate level, i.e., 30 to 60 wafers per hour per beamline. Although electron beam and focused ion beam techniques can meet critical dimension requirements, their low production rates make them unsuitable for high rate production. X-ray lithography was successfully utilized in 1989 to demonstrate the fabrication of functional NMOS and CMOS circuits with fully scaled 0.5 micron ground rules. The utilization of an electron storage ring (ESR) as the X-ray source, with its highly collimated X-ray flux at high intensity levels that can be delivered to many ports, has the necessary properties to satisfy both resolution and production volume needs of the future.
The major subunits of an ESR based X-ray lithography system (XLS) include a preaccelerator, beam transport line, electron storage ring (synchrotron), lithography beamlines and exposure stations (aligner/stepper), where the actual lithography takes place. The lithography beamline performs a specialized role in the lithography system in that it connects the source to the lithography station. One basic ESR XLS performance requirement is to support 0.25 micron resolution lithography with the given stepper. One of the resolution related optical parameters is the lithography exposure window. The elements that form the exposure window in the beamline and the stepper include mirrors, filters and an exit window. The function of the exit window is to separate the beamline from the stepper (lithography chamber) and contribute to the formation of the exposure window. The lithography beamline connects the synchrotron to the exposure chamber. The beamline operates at an ultra high vacuum (UHV) while the exposure chamber operates at atmospheric pressure. In order to ensure the vacuum integrity of the beamline, the exit window must be able to withstand this pressure differential. The window material must also be transmissive to a desirable beam energy. This means the exit window material must behave as a high pass filter by providing minimum attenuation in that portion of the spectrum which is required for lithography, i.e. 800-1800 eV range, and maximum cutoff in the low energy interval of radiation. This absorption can be minimized with properly selected window materials and optimized material thickness. Beryllium is one of the possible exit window materials. The ideal window will have minimum power absorption in the exposure spectrum range and, therefore, minimum thickness. However, this minimum thickness will not be able to withhold the 1 Kg/cm.sup.2 atmospheric pressure differential. It is clear that the power transmission and the mechanical strength of the window are competing factors that must be optimized.
One basic future production lithography requirement dictates the manufacture of larger chip sizes with horizontal and vertical dimensions of 50 mm by 25 mm or larger. This defines the area to be illuminated with X-rays. The most practical way currently known in the prior art is to generate a "flat" (a few mm in the vertical dimension) X-ray beam with the required width (50 mm or wider) and to scan this beam over the field or move the wafer relative to the beam. This scanning type exit window is slightly larger than the beam cross section and it is synchronously moved with the scanning beam. The scanning exit window can be comprised of a flat beryllium sheet 4 mm.times.60 mm and 18 microns thick and will withstand the required pressure differential as well as be transmissive to the desired beam energy. However, the scanning exit window is a complicated subsystem of the beamline and makes the fabrication of the beamline expensive and its operation difficult. In addition, scanning the beam introduces difficulties in the optics of the lithography beamline including mechanical movement that generates uncompensated vibration. Further disadvantages include fatigue of connecting bellows which may trigger a major vacuum accident, higher thermal density load requiring additional cooling and an additional control system is required. Thus, there is a need to develop a stationary exit window that meets the requirements of pressure differential and beam energy transmission.
Furthermore, there is a need to develop a stationary window that eliminates the need for moving parts and that is easier and less costly to maintain.